Monohull yachts are provided with fixed or retractable keels, the purpose of which is to enable the yacht to beat to windward with a minimum amount of heel. By minimising heeling of the yacht, the underwater hull is maintained as close to symmetrical as possible and thus the smallest rudder angle is necessary to prevent the yacht rounding up. By maintaining the rudder angle at a minimum, drag is minimised. Further when a yacht is substantially upright, the sail is operating at a maximum efficiency. Maintaining a yacht upright is also important in cruising yachts where a large crew is not available and comfort is important.
A keel additionally has the effect of reducing the amount of drift of the yacht to leeward when beating to windward. A keel further has the advantage of permitting the boat to carry the maximum amount of sail for a given wind strength for maximum speed. Conventional keels however have a number of disadvantages. In particular keels require a yacht to heel before the righting moment of the keel comes into effect. Keels also usually substantially increase the draft of a yacht which therefore, particularly in larger yachts, limits the waterways in which the yacht may be used. Whilst some yachts can be provided with retractable centreboards or swing keels which may be weighted, this is a compromise which is not suited in all applications and can have the disadvantage of comprising accommodation within the yacht.
To overcome some of the above disadvantages of conventional keels, some of the large yachts are provided with a canting keel which is mounted for pivotal movement relative to the longitudinal centreline of the hull, the keel usually including a weighted bulb at its lower end to provide maximum righting movement and having an upper end which projects into the hull. Whilst such keels have proven to be effective in use and improve the speed of yachts, their design and implementation has to address significant structural, safety and cost problems. The canting keel also does not prevent sideways drift to leeward and therefore an additional dagger board or canard is often required.
A yacht may be provided with a movable water ballast which overcomes the mechanical disadvantages of a canting keel but has the disadvantage of increasing the wetted surface of the yacht hull. Furthermore as the water ballast is wholly within the hull, it is less effective that a canting keel in that it has a smaller righting moment for its weight.
Another proposal has been to use a movable weight within a hull which is moved to provide the require righting moment depending upon the manner in which the yacht is sailing however such as arrangement is constrained by the hull extremities and has inadequate righting moment.
Multihull vessels overcome the problem of having a large keel depending from a hull or hull of a watercraft however multihulls have the disadvantage that they have excessive beam which makes it difficult for them to be accommodated in marinas at a reasonable cost.